Add "empty" class for Image module when path/exec is returning an empty value.

man/waybar-image.5.scd

@@ -87,3 +87,4 @@ $path\\n$tooltip
 # STYLE
 
 - *#image*
+- *#image.empty*

src/modules/image.cpp

@@ -63,9 +63,11 @@ auto waybar::modules::Image::update() -> void {
     }
     image_.set(pixbuf);
     image_.show();
+    image_.get_style_context()->remove_class("empty");
   } else {
     image_.clear();
     image_.hide();
+    image_.get_style_context()->add_class("empty");
   }
 
   AModule::update();